As for the shape of an insert seat of a cutting tool, to which a cutting insert is attached, it has been typically known that a curve portion is formed between a bottom surface and a wall surface of the insert seat. A conventional cutting tool adopting the structure is exemplified by that disclosed in Patent Literature 1. A cutting tool disclosed in Patent Literature 1 is a cutting tool to which a cutting insert is removably attached, and has a cutout groove having an arcuate cross section, that is, a curve portion formed at a crossing portion between a bottom surface and a wall surface of an insert seat. The curve portion is formed in order to alleviate a stress on the crossing portion between the bottom surface and the wall surface so as to prevent a crack. In view of this, in Patent Literature 1, the curve portion is formed in such a manner as to extend toward the crossing portion from the outer end of the insert seat where a crack is liable to occur. Thus, the curve portion becomes deeper at a portion near the outer end whereas it becomes gradually shallower toward the back.